A three-dimensional system for the growth of human tissues in vitro has been developed in our laboratory. In this physiological method stromal cells are established on a support matrix and act to provide a foundation for the continued growth and differentiation of cells. Histologically complete bone marrow and skin have been maintained in long-term culture using this technique. The human skin model consists of a dermal layer of fibroblasts and naturally secreted collagen and an epidermal layer of melanocytes and keratinocytes. A dermal/epidermal junction is present. The human bone marrow culture system is the first in vitro method which supports the proliferation of erythroid, myeloid, and lymphoid cells simultaneously and exhibits active hematopoietic stem cell growth. Support of this grant will allow the further characterization and modification of this system for use in cytotoxicity assays, engraftment studies, and in the in vitro study of various disease etiologies. The neutral red assay will be employed as a cytotoxicity method for screening various toxic substances on our skin and bone marrow substrates. These three-dimensional substrates will also be used to evaluate the mutagenic effects of various test agents.